Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM)-type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements, sometimes referred to as the gas diffusion media components, that: (1) serve as current collectors for the anode and cathode; (2) contain appropriate openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts; (3) remove product water vapor or liquid water from electrode to flow field channels; (4) are thermally conductive for heat rejection; and (5) have mechanical strength. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (e.g., a stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the MEA described earlier, and each such MEA provides its increment of voltage.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
Examples of technology related to PEM and other related types of fuel cell systems can be found with reference to commonly-assigned U.S. Pat. No. 3,985,578 to Witherspoon et al.; U.S. Pat. No. 5,272,017 to Swathirajan et al.; U.S. Pat. No. 5,624,769 to Li et al.; U.S. Pat. No. 5,776,624 to Neutzler; U.S. Pat. No. 6,103,409 to DiPierno Bosco et al.; U.S. Pat. No. 6,277,513 to Swathirajan et al.; U.S. Pat. No. 6,350,539 to Woods, III et al.; U.S. Pat. No. 6,372,376 to Fronk et al.; U.S. Pat. No. 6,376,111 to Mathias et al.; U.S. Pat. No. 6,521,381 to Vyas et al.; U.S. Pat. No. 6,524,736 to Sompalli et al.; U.S. Pat. No. 6,528,191 to Senner; U.S. Pat. No. 6,566,004 to Fly et al.; U.S. Pat. No. 6,630,260 to Forte et al.; U.S. Pat. No. 6,663,994 to Fly et al.; U.S. Pat. No. 6,740,433 to Senner; U.S. Pat. No. 6,777,120 to Nelson et al.; U.S. Pat. No. 6,793,544 to Brady et al.; U.S. Pat. No. 6,794,068 to Rapaport et al.; U.S. Pat. No. 6,811,918 to Blunk et al.; U.S. Pat. No. 6,824,909 to Mathias et al.; U.S. Patent Application Publication Nos. 2004/0229087 to Senner et al.; 2005/0026012 to O'Hara; 2005/0026018 to O'Hara et al.; and 2005/0026523 to O'Hara et al., the entire specifications of all of which are expressly incorporated herein by reference.
A fuel cell subgasket can be used for the following functions: (1) because the ionomer membrane is a soft material that drastically changes dimensions with temperature and relative humidity, subgasket provides a tough and dimensionally stable material (e.g., PET or PEN) to seal regions of the membranes; (2) because the ionomer membrane is soft, the use of a subgasket allows for increased seal loads without damaging the membrane; (3) edges of different components in the PEM fuel cell can cause local stress concentrations on the membrane (e.g., the use of subgaskets can prevent these edges from causing premature membrane failure); and (4) the location of the subgasket edge can be used to control the activity at the catalyst edges (e.g., if the subgasket is impermeable, it can prevent reactants from reaching the catalyst, thus the subgasket edge can control the effective active edge of the catalyst). Significant improvements in MEA life have been demonstrated by artificially controlling catalyst edges using a subgasket.
However, conventional methods of attaching the subgasket to the ionomer membrane have not been completely satisfactory. Various attempts have been made to overcome this problem. For example, some manufacturers have used hot pressure to attach the subgaskets to the ionomer membranes. In addition to the heat and pressure required in the technique, an additional disadvantage is that the catalyst layers are added after the subgasket. This prevents the subgasket from being attached over the catalyst layer. The subgasket can be attached under the catalyst in this method, but this will often lead to catalyst cracking and delaminating at the subgasket edge. This cracking results in a fuzzy catalyst edge at the subgasket edge. In order to offset the anode and cathode catalyst edges, this method requires additional space to account for the uncertainty in the catalyst edge caused by the cracking. Additionally, this method could leave gaps of exposed ionomer membrane between the catalyst and subgasket edges.
Other attempts use an approach where the subgasket is placed on top of the membrane. Then this three layer structure is sandwiched between two pieces of catalyst coated diffusion media. The entire assembly is then hot pressed past the glass transition point of the ionomer to form the MEA. While this approach is fairly robust in terms of controlling catalyst edges it has several disadvantages. First, the heat and pressure required to get the bond can cause the ionomer membrane to flow, which can lead to thinning under the subgaskets. Second, the heat/cool cycles can induce thermal stresses in the part. Failures can then occur at the subgasket edge. Third, because the entire MEA, including the gas diffusion medium (GDM), is assembled in one hot press step, it is very difficult to then inspect the catalyst edge positions.
Accordingly, there exists a need for new and improved methods for attaching the subgasket to the ionomer membrane, wherein the methods provide for the precise location of the subgasket relative to the other component edges of the fuel cell so as provide the functionality required to extend the ionomer membrane life and prevent damage to the ionomer membrane during the assembly process.